This invention relates to pulse signalling systems of the code type and more particularly to an improved autocorrelation technique for use in such pulse signalling systems.
Correlation techniques have been utilized in the past in signal processing systems employing signals in the form of a pulse or sequence of pulses. Such pulse signalling systems include, for example, radiant energy reflecting systems, such as radar, radio range finders, radio altimeters, and the like; pulse communication systems, such as over-the-horizon systems employing various types of scatter techniques, satellite communication systems and the like; and multiple access systems employing address codes to enable utilization of the multiple access system. Correlation techniques when employed in radiant energy reflection systems enhance the resolution of closely spaced reflecting surfaces and in addition, particularly when wide pulse widths are employed, increase the average power transmitted. Correlation techniques employed in pulse communication systems result in increased signal-to-noise ratios without increase of peak transmitter power and minimize multiple paths effects (fading). Correlation techniques when employed in a multiple access environment also result in increased signal-to-noise ratio without increase of transmitter power and if properly coded prevents or at least minimizes the interference or crosstalk between one or more address codes.
According to prior art correlation techniques the received signal is processed by obtaining the product of code elements of the received signal and code elements of a locally generated signal of the same waveform and period as the received signal and integrating the resultant product. The optimum output for such a correlation would be a single peak of high amplitude which has a width substantially narrower than the pulse width of the received signal. Most correlation systems in use today do not produce the desired optimum waveform, but rather provide an output whose waveform has spurious peaks in addition to the desired high amplitude peak. The presence of these spurious peaks is undesirable in that the resolving power of radiant energy reflecting systems is reduced, the signal-to-noise ratio of pulse communication systems and multiple access systems and the minimization of multiple path affects of pulse communication systems is reduced to a level below the optimum value.
Previously a number of improved correlation techniques have been proposed that will result in an impulse correlation function. The term "impulse correlation function", and more specifically "impulse autocorrelation function", as employed herein, refers to a waveform having a single high amplitude peak completely free from spurious peaks of lower amplitude elsewhere in the waveform.
One of these proposed improved autocorrelation techniques which is most closely related to the present invention is fully disclosed in the copending application of F. S. Gutleber, Ser. No. 645,697, filed June 13, 1967, now U.S. pat. No. 3,519,746. This copending application discloses a class of codes including two codes, termed code mates, where the code mates have cooperating autocorrelation functions so that when they are autocorrelation detected, and the resultant detected outputs are linearly added there is provided an impulse autocorrelation function having an impulse output at a given time and a zero output at all other times. The code mates generated are time or frequency multiplexed for transmission to the detector to provide long code sequence to increase the average transmitting power. The transmitted multiplexed code mates are separated consistent with the type of multiplexing being employed prior to autocorrelation detection and linear addition. The number of code mates can be increased in accordance with said copending application by interleaving each of the original code mates and the different time displaced versions thereof to provide a plurality of first codes and interleaving one code mate and the complemented version of the other code mate of each of the original code mates and the different time displaced versions thereof to provide a code mate for each of the first codes. This process of increasing the number of code mates can be continued repeatedly with the newly generated code mates to further increase the number of code mates.
The codes forming the code mates of said copending application require a complete code fill-in to work, that is, a code digit of either binary condition had to be present in each digit time slot of the repetition period of the code. Thus, the codes had to be repeated and the detection process required multiplying and integrating over N code digits for codes of length N.